At least where I am in the world, you can get the nutrition data on the shit they sell in fast food restaurants from either their own website or from the store itself.

If you're going to "go nuts" on a cheat day I'd highly recommend going for high carb OR high fat (high carb better if your normal cutting diet is low in carbs). If your blow out has both at once then you'll be a.) eating more calories and b.) re-filling the fat cells you'd worked so hard to empty during the week.

When having a cheat meal, work out whether you really need to eat 4,000 calories of food to be satisfied that you've cheated, or whether you just need a small burger to psychologically improve the boredom of dieting.

The fact that you need to ask where and how to work out what the nutrition content of junk food is should highlight that it's not the best choice if you're cutting.

I would highly recommend getting one of these: www.amazon.com/AccuFitness-Accu-Measure-Body-Fat-Caliper/dp/B000QURRUK

They are not expensive and will help you keep an accurate track of your skin fold measurement. I wouldnt worry too much about what % it says you are but keeping track of how large the skinfold is, say 20mm, and then a month later its 18mm then you can be sure you are making progress.